Outlook PST and OST files

OST file is similar to PST file, but it is only created and used by Outlook if you are using a Microsoft Exchange Server account to manage contacts, e-mail and time scheduling.

The size of these files are limited by the default settings of Outlook. The PST/OST file size is different for various Outlook versions. If you get too many e-mail messages with large e-mail attachments you can increase the PST file limit.
On the other hand, on slower machines, or small hard drive, you can decrease the size of PST file for better computer performance. Also smaller files are easier to backup.

There are two types of PST/OST files. One being ANSI and the other UNICODE.

MaxLargeFileSize (MaxFileSize) entry sets the absolute maximum file size that both the PST and OST files can grow to on the storage.

WarnLargeFileSize (WarnFileSize) entry sets the maximum amount of data that both PST and OST files can have inside then. After the maximum data limit is reached, no further email data will be added. However the size of the actual file may still increase because of other internal Outlook processes.

How to change size of PST or OST file in the Windows Registry

In tables listed below you will see the default size limits for both ANSI and UNICODE formatted OST files in various Outlook versions.

Default PST/OST file size settings in Outlook 2003 and Outlook 2007

Name

Type

Valid Data Range

Default

MaxLargeFileSize

REG_DWORD

0x00000001 – 0x0000C800

0x00005000 20,480 (20 GB)

WarnLargeFileSize

REG_DWORD

0x00000000 – 0x0000BE00

0x00004C00 19,456 (19 GB)

MaxFileSize

REG_DWORD

0x001F4400 – 0x7C004400

0x7BB04400 2,075,149,312 (1.933 GB)

WarnFileSize

REG_DWORD

0x00042400 – 0x7C004400

0x74404400 1,950,368,768 (1.816 GB)

Default PST/OST file size settings in Outlook 2010 and Outlook 2013

Name

Type

Valid Data Range

Default

MaxLargeFileSize

REG_DWORD

0x00000001 – 0x0000C800

0x0000C800 51,200 (50 GB)

WarnLargeFileSize

REG_DWORD

0x00000000 – 0x0000BE00

0x0000BE00 48,640 (47.5 GB)

MaxFileSize

REG_DWORD

0x001F4400 – 0x7C004400

0x7BB04400 2,075,149,312 (1.933 GB)

WarnFileSize

REG_DWORD

0x00042400 – 0x7C004400

0x74404400 1,950,368,768 (1.816 GB)

If you need to change PST size limits for UNICODE PST file (current versions of Outlook):

Open the Windows Registry Editor (click to Windows search box and type regedit) and navigate to:

The warn size for UNICODE PST files is also set in megabytes and it is about 5% smaller than physical size of PST file. For example 20 480 MB of physical size is 19 456 MB of warn size (20 480*0,95=19 456). Use Windows Calculator → compute the warn size and type it to Value data box → Click on OK button.

Registry editor how to edit DWORD Value data for WarnLargeFileSize

Now you have to set the maximal size of Outlook PST/OST files in current UNICODE file format. The maximal recommended size of UNICODE PST file is 50 GB, but it can be larger than that.

Registry editing complete

If you need to change size for older PST files in ANSI format, you can use the same instructions, but registry entries are called MaxFileSize and WarnFileSize instead.
The size of ANSI is calculated in bytes, for example 2 GB = 2 * 1024*1024*1024 = 2 147 483 648 bytes. The maximum file size is 2 GB, larger sizes will be ignored.

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